Scrubber systems have been developed for controlling harmful emissions from industrial and utility boilers like those used in power plants. Such scrubber systems are generally of either the dry or wet types. Dry scrubber systems generally include an open chamber in which the flue gas is directed through a liquid spray of lime and fly ash slurry. A reaction occurs with the sulfur dioxide in the gas to form a calcium compound in dry particulate form which can then be collected at the outlet of the chamber, thereby "scrubbing" the flue gas free of sulfur dioxide pollutants.
On the other hand, in the so-called "wet scrubbers", the sulfur dioxide is not collected in dry particulate form, but rather is collected in the form of a slurry in a tank of aqueous absorbent for periodic removal in liquid slurry form. In order to avoid excessive maintenance costs from the build-up of deposits of fly ash, the fly ash must first be removed by means of filtration or electrostatic precipitation before the flue gas can be treated in a wet scrubber. This results in two large, expensive structures. Further, since the fly ash is typically removed separately, the alkalinity which otherwise would have been provided by the presence of fly ash is not present, and thus some of the fly ash which was removed must then be added to the aqueous absorbent, which of course adds further expense and complication.
U.S. Pat. No. 3,836,630 shows a system wherein sulfur dioxide absorption and oxidation is carried out in separate vessels.
U.S. Pat. Nos. 4,156,712 and 4,229,417 disclose a method and apparatus for removing sulfur dioxide from flue gases by simultaneous absorption and oxidation in an aqueous absorbent within a single vessel. As the liquid absorbent in the vessel is agitated, flue gas is sparged into the upper region as oxygen-containing gas is sparged into the lower region of the body of liquid absorbent. Alkaline material is introduced into the body of liquid absorbent that fixes the sulfur dioxide in the form of a sulfate which then precipitates out in solution for removal. These references suggest that gas/liquid contacting and solid (fly ash) precipitation can be carried out in the same device, however, since the flue gas is introduced into the upper region of the liquid absorbent via perforate or notched pipes, at least some prescrubbing of the fly ash particulate therefrom is required to avoid clogging.
My U.S. Pat. No. 4,762,686 is directed to a system for simultaneous fly ash and sulphur dioxide removal, but which does not incorporate an existing baghouse or precipitator upstream from the wet scrubber module.
A need has thus arisen for a retrofit flue gas scrubber system of the wet scrubber type for use with a scrubber system having an existing baghouse or precipitator, so that residual particulate removal and primary sulfur dioxide removal can be accomplished more efficiently.